This invention relates to an internal combustion engine having a spiral shaped intake port.
Such a spiral intake port is used to produce swirls of intake air in a combustion chamber in a cylinder of an internal combustion engine.
FIGS. 1 and 2 show such a conventional spiral intake port as disclosed on page 44 of the magazine "Service Shuho" No. 436 published by Nissan Motor Company Limited in March 1981. An intake valve 2 and an exhaust valve 3 face a combustion chamber of a cylinder 1. The axis of the intake valve 2 and the axis of the exhaust valve 3 are positioned at the plane X passing through the central axis 0 of the cylinder 1. A spiral intake port 5 is formed in a cylinder head 4 of the cylinder 1.
The spiral intake port 5 is formed such that it turns around an intake valve shaft 2a of the intake valve 2 in a spiral shape. The spiral port 5 has a turning portion 5A and an approach portion 5B. The approach portion 5B is positioned at one side of or out of the plane X. The intake passage of the turning portion 5A extends to intersect the plane X and passes beyond the plane X from said one side to the other side thereof in a spiral shape. The axis Y of the major intake air in the approach portion 5B at an entrance thereof extends to intersect the outer wall 5o of the turning portion 5A.
In such a conventional spiral intake port, the majority of intake air is turned by means of the outer wall 5o of the turning portion 5A after it has passed the approach portion 5B. After the intake air directly flows into the cylinder 1, the large swirls are produced along the inner surface of the cylinder 1.
For such a reason, at a piston intake stroke, large swirls are produced along the inner surface of the cylinder 1 while some vacuum portion occurs at the center of the cylinder 1. Such vacuum portion is apt to prevent the swirls from remaining during piston compression stroke.
Thus, after an ignition plug 8 sparks the combustion speed decreases so that the flame propagation speed becomes low.